1. Field of the Invention
The present invention relates to a color cathode ray tube, and more particularly to a color cathode ray tube, wherein a thickness of an effective surface of a panel is changed, which can reduce mislanding caused by thermal expansion of a shadow mask, vibration, and impact.
2. Description of the Related Art
Generally, in a color cathode ray tube, a shadow mask is provided opposite a fluorescent screen, which comprises three color fluorescent layers. Then, three electron beams emitted from an electron gun are selected by the shadow mask to be introduced into the three color fluorescent layers, thereby displaying a color image on the fluorescent screen.
FIGS. 1A and 1B show a structure of the main parts of a cathode ray tube. Normally, the color cathode ray tube comprises a panel 2, having a substantially rectangular effective region, whose inner and outer portion are curved. A fluorescent screen 3 having three color fluorescent layers is formed on the curved inner surface of the effective surface 1. A shadow mask 4 comprises a mask body 5 and a mask frame 6 whose shape is substantially rectangular. The mask body 5 has a substantially rectangular effective surface having apertures through which electron beams pass are formed in a curved portion so as to correspond to the inner surface of the panel 2. The mask frame 6 is attached to a peripheral portion of the mask body 5. The shadow mask 4 is supported by the inner side of the panel by inserting a stud pin 8 formed in the panel 2 into an elastic support 7 attached to the mask frame 6.
FIGS. 1A, 1B and FIG. 3 show the structure in which the belt-like elastic support 7 is attached to the central portion of each side of the mask frame 6 to support the shadow mask 4. However, there may be provided a structure in which a wedge-shaped elastic support is attached to each diagonal portion of the mask frame 6 to support the shadow mask.
In the above-structured color cathode ray tube, in order to display an image having no degradation of color purity on the fluorescent screen, it is required that three electron beams, which are passed through the respective apertures of the shadow mask, be correctly landed onto the three color fluorescent layers constituting the fluorescent screen 3, respectively. In order to improve the color purity, a positional relationship between the panel 2 and the shadow mask 4, particularly a distance (value q) between the inner surface of the panel 2 and the effective surface of the shadow mask 4, must be kept to have a predetermined allowable range.
However, the mask body 5 of the normal shadow mask 4 is formed of a thin carbon steel plate, and the amount of electron beams which reaches the fluorescent screen 3 through the apertures formed in the effective surface is 1/3 of the amount of electron beams emitted from the electron gun. Most of the electron beams collide against the shadow mask. As a result, the shadow mask 4 is heated and thermally expanded, and particularly, the curved shaped mask body 5 having a thin plate thickness is expanded in three directions of the fluorescent screen 3 causing doming. If the amount of expansion due to the doming exceeds the allowable range of value q, mislanding of the electron beams onto the three color fluorescent layers is caused, resulting in degradation of color purity is generated. The degree of the degradation of color purity, which is generated by the thermal expansion of the shadow mask, is different depending on the amount of flow of the electron beams, the size of the image pattern, and display duration of the image pattern.
Regarding the mislanding generated by the thermal expansion of the shadow mask 4, particularly, after a long period of time when the mask body 5 is heated at the initial stage when the operation of the color cathode tube is started, a temperature of the mask body 5 is transmitted to the mask frame 6 to obtain a thermal equivalence state, in other words, mislanding, which is generated for a period of time (about 30 minutes) until the temperature of the mask body 5 and that of the mask frame 6 are substantially the same, Japanese Patent Application KOKAI Publication No. 44-3547 discloses as follows.
A bimetal element is provided between the mask frame 6 and the elastic support 7 for supporting the shadow mask 4, thereby an effective correction can be performed. However, in a case where a high luminance image is locally displayed for a relatively short period time, local expansion is generated. Due to this, mislanding generated by such the local expansion cannot be corrected by the provision of the bimetal element therebetween.
Regarding the mislanding generated by the thermal expansion of the shadow mask 4, a rectangular pattern is generated on the fluorescent screen by a signal generator. Then, the shape and position of the rectangular pattern are variously changed to measure the degree of the mislanding. As a result, as shown in FIG. 2A, in a case where a rectangular pattern 10a having a large current and high luminance is generated in substantially the whole area of the fluorescent screen 3, the degree of the mislanding is small. However, as shown in PIG. 2B, if an elongated rectangular pattern 10b having the large current and high luminance is generated close to the center from the right end or the left end of the fluorescent screen 3 and extends along a vertical axis, i.e., Y-axis, the largest degree of the mislanding is generated.
The above can be easily understood from the following explanation.
First, the cathode ray tube is generally designed such that an average anode current to be added to the cathode ray tube, that is, a current flowing in an anode, does not exceed a fixed value in the entire screen, the amount of the beam current colliding against the shadow mask per unit area in the case where rectangular pattern 10a having a high luminance is generated as shown in FIG. 2A is smaller than the case of FIG. 2B, and the rise of the temperature of the shadow mask is relatively low.
Secondly, regarding the pattern having a local high luminance, as shown in the elongated rectangular pattern 10b of FIG. 2B, even if the shadow mask is thermally expanded, in the case where the local high luminance pattern is generated in the central portion of the fluorescent screen 3, the mislanding is not easily generated since a deflection angle of the electron beam is small. However, the extent that the thermal expansion of the shadow mask appearing as mislanding increases as the portion where the pattern is generated is moved from the center to the right and left ends. However, in the case where the pattern is generated at the right and left ends of the screen 3, since the mask body 5 is fixed by the mask frame, the doming caused by the thermal expansion becomes small. In the end, the largest mislanding is generated in the case that the pattern having a high luminance is generated in the portion close to the center from the right and the left ends of the screen 3.
FIG. 3 shows mislanding in a case where the high luminance pattern is generated at the portion close to the center from the right and left ends of the screen 3. In this case, the shadow mask 4 is supported by inserting the stud pin 8 formed in the panel 2 into the elastic support 7 attached to the mask frame 6. The effective surface of the mask body 5, on which a large number of apertures are arranged, is opposed to the fluorescent screen 3 formed in the inner surface of the panel 2, and the shadow mask 4, which is shown by a solid line, is used as a shadow mask, which is placed at a normal position. When the shadow mask is placed at the position shown by the solid line, an electron beam 13, which is passed through one aperture 12 positioned at slightly central portion from the right and left ends of the shadow mask 4, is landed onto a correctly corresponding fluorescent layer 14. However, if the high luminance image is displayed by the electron beams having the large current passing in the vicinity of the aperture 12, the portion in the vicinity of the aperture 12 is locally thermally expanded as shown by a one-dot broken line, an electron beam 13a passing through the aperture 12a displaced by the thermal expansion is not landed onto the predetermined fluorescent layers 14.
Particularly, the latest color cathode ray tube whose effective portion of the panel is flattened has been mainly used, and the effective surface of the shadow mask of the mask body has been also flattened in accordance with the flatted effective portion of the panel. Due to this, such a flattened shadow mask is easily deformed by the thermal expansion causer by collision of the electron beams, and the mislanding is largely generated.
Regarding the color cathode ray tube whose effective portion of the panel is flattened, Japanese Patent Application KOKAI Publications Nos. 61-163539 and 61-88427 disclose a structure in which the shape of the shadow mask is changed to control the mislanding. However, in the color cathode ray tube in which the flattened panel and the flattened shadow mask are combined, a sufficient technical advantage cannot be obtained by the shape of the shadow mask disclosed in the above publications.
In other words, in the latest color cathode ray tube, the panel and the shadow mask are more flattened than those disclosed in the above publications. Due to this, the mislanding caused by the thermal expansion of the shadow mask caused by the collision of the electron beams is large. Therefore, it is required that a mechanism for correcting such a large mislanding be provided. However, there is a problem in that such a large mislanding cannot be sufficiently corrected in the shape of the panel and the shadow mask disclosed in the above publications.
In order to deal with such a problem, Japanese Patent Application KOKAI Publications Nos. 61-163539 and 61-88427 disclose the structure in which the curved surface of the panel is changed to control the mislanding generated by the thermal expansion of the shadow mask.
However, even if the curved surface is redesigned as disclosed in the above publications, no advantage is achieved in a flat panel recently put to practical use and having a substantially spherical surface which reflect a natural ambient image applied onto it from the outside.
Moreover, regarding the color cathode ray tube in which the panel and the effective surface of the shadow mask are flattened, the following problems exist in addition to the thermal expansion of the shadow mask.
More specifically, in the mask body of the shadow mask of the color cathode tube having the flatten effective portion of the panel, there is used a material having a low coefficient of thermal expansion, such as invar, other than the low carbon steel plate used in the shadow mask of the normal color cathode ray tube. The normal mask body of the normal shadow mask is formed to have a predetermined curve surface by press-molding after apertures are formed by photoetching. In the mask body having a high curvature, the mask body is sufficiently plastically deformed at the time of press-molding, so that the necessary mechanical strength can be provided thereto. However, the flatten mask body cannot be sufficiently plastically deformed, and a portion having low mechanical strength is locally formed in the mask body. In other words, in the flatten mask body, an amount of processing at the time of press-molding and an amount of elongation are decreased, and there is generated a portion which cannot be formed in the plastically deforming area and stays in the elastically deforming area. Due to this, the portion having low mechanical strength is locally formed in the mask body. In the shadow mask whose effective surface is substantially rectangular, the portion having low mechanical strength appears in the vicinity of the long axial end close to the central portion from the short side positioned in the long axial direction separating from the center rather than the long side positioned in the direction of the short axis (vertical axis) to the center.
In other words, the area close to the central portion from the short side is far from the center of the shadow mask and not surrounded with a skirt portion unlike the diagonal axial end portion. Due to this, such the area cannot be sufficiently plastically deformed at the time of press-molding, and the processing of the above area stays in the elastically deforming area. As a result, the above area cannot be formed to have a predetermined curved surface, the mechanical strength of the area becomes low, and the area is deformed by impact. Moreover, if vibration or impact is added thereto, there are generated problems in which the area easily resonates, and degradation of color purity occurs.
As mentioned above, in order to display the image having no degradation of color purity on the fluorescent screen of the color cathode ray tube, the distance between the inner surface of the effective portion of the panel and the effective surface of the shadow mask must be kept to a predetermined allowable range. However, most of the electron beams, which are emitted from the electron gun, collide against the shadow mask. The shadow mask 4 is thermally expanded in the direction of the fluorescent screen by the collision of the electron beams. As a result, the electron beams are mislanded onto the three color fluorescent layers, and the purity of color is degraded. There are two mislanding generated by the thermal expansion, that is, mislanding generated for a relatively long period of time until the mask body and the mask frame are in a thermal equivalence state from the initial stage when the operation of the color cathode tube is started, and a local mislanding generated when a high luminance image is locally displayed for a relatively short period time. Among these, in the case of the mislanding generated for a relatively long period time from the initial stage when the operation of the color cathode tube is started, the mislanding can be effectively corrected by providing the bimetal element between the mask frame and the elastic support for supporting the shadow mask 4. However, in the case of the mislanding generated when the high luminance image is locally displayed for a relatively short period time, the mislanding cannot be corrected by providing the bimetal element therebetween, and this local mislanding appears at the largest degree when a high luminance image is generated at a portion closer to the central portion than the right and left ends.
The above mislanding generated by the thermal expansion of the shadow mask is easily generated in the latest color cathode ray tube whose panel and shadow mask are flattened. Due to this, in the color cathode ray tube whose panel and shadow mask are flattened, there has been known a method for changing the shape of the curved surface of the panel and shadow mask, thereby preventing the mislanding. However, in the shape of the well-known shadow mask, the technical advantage cannot be sufficiently obtained. However, even if the curved surface is changed, the sufficient technical advantage cannot be obtained in the flat panel having a substantially spherical surface, which has been recently used in practical such that the natural ambient image is reflected from an outer surface of the panel despite the ambient light applied onto it.
Moreover, in the color cathode ray tube in which the effective surface of the shadow mask is flatten, the mask body cannot be sufficiently plastically deformed at the time of press-molding, so that a portion having low mechanical strength is locally formed in the mask body. The lowest mechanical strength appears in the vicinity of the horizontal axial end close to the central portion from the short side of substantially the rectangular shadow mask. Then, this portion is deformed, or resonates by vibration or impact, and degradation of color purity occurs.